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ABOUT THIS IMAGE:

This photo mosaic, which shows a field of distant galaxies, is a
computer enhanced reproduction of a picture taken 4 March 1994
with the repaired Hubble Space Telescope. It combines 16 exposures of
15 minutes each, taken through two filters (F555W and F814W) with the
Wide Field Planetary Camera 2. The HST WFPC2 field is chevron-shaped,
because it is a mosaic of images recorded with three Wide Field cameras
and one higher resolution camera (Planetary Camera) in the upper left.

The brightest object in this picture is NGC 4881, approximately
centered here in the Planetary Camera (the small quadrant). It is a
13th-magnitude elliptical galaxy in the outskirts of the Coma Cluster,
a great cluster of galaxies more than 5 times farther away than the
Virgo Cluster. The radical velocity (redshift) of NGC 4881, based on
the Doppler displacement of lines in its spectrum, is about 7000
km/sec. Except for a 16th-magnitude Coma spiral at the right and a few
foreground stars of the Milky Way, nearly everything else in this field
lies far beyond the Coma Cluster. There is a fascinating assortment of
background galaxies, including an apparent galaxian merger in
progress.

Purpose: This HST-WFPC2 observation was made to explore the use the
globular star clusters surrounding NGC 4881 as distance indicators for
inferring the distance to the Coma Cluster. They are barely visible
point sources in this reproduction. The distance to the Coma Cluster
is an important cosmic yardstick for scaling the over all size of the
universe, because Coma (unlike Virgo) is far enough away that regional
departures from a smooth expansion of the universe should not be a
major source of uncertainty if Coma is used for estimating the age and
rate of expansion (the Hubble Constant).

The brightness distribution of globular clusters has been studied in a
number of nearer galaxies. They are most numerous between -7 and -8
absolute magnitude. In the Milky Way they peak at -7.6 absolute
magnitude. We must find that peak ("turnover") in NGC 4881 in order to
judge the distance. Within statistical uncertainties, the number of
globulars per magnitude in NGC 4881 increases down to our present
threshold of 27.6 magnitude. We do not yet see evidence of the
turnover in NGC 4881, which suggests that the Coma Cluster may be more
than 100 megaparsecs away and that the Hubble Constant may therefore be
less than 70 km/sec per megaparsec. The adding together of more
exposures will evidently be needed to reach a fainter threshold and
find the turnover. Though not yet definitive in itself, this
exploratory observation of NGC 4881 shows that it is within the reach
of HST to obtain a definitive globular-cluster distance to the Coma
Cluster and an associated value of the Hubble Constant.